Method for producing 4-vinylcyclohexene, ethyl benzole and styrene

ABSTRACT

The invention relates to a process for preparing 4-vinylcyclohexene, which comprises the steps  
     (A) providing an n-butane-containing feed gas stream,  
     (B) feeding the n-butane-containing feed gas stream into at least one dehydrogenation zone and dehydrogenating n-butane to butadiene to give a product stream comprising butadiene, n-butane, possibly 1-butene and 2-butene and possibly water vapor and other secondary constituents,  
     (C) feeding the product stream from dehydrogenation, if appropriate after separating off water vapor and secondary constituents, into a dimerization zone and catalytically dimerizing butadiene to give a product stream comprising 4-vinylcyclohexene, n-butane and possibly 1-butene, 2-butene and unreacted butadiene, and  
     (D) separating off 4-vinylcyclohexene from the product stream from the dimerization and recirculating n-butane and possibly 1-butene, 2-butene and unreacted butadiene to the dehydrogenation zone.

[0001] The present invention relates to a process for preparing4-vinylcyclohexene and for preparing ethylbenzene and styrene asdownstream products of 4-vinylcyclohexene.

[0002] It is known that 4-vinylcyclohexene can be prepared bycyclodimerization of 1,3-butadiene in the liquid phase in the presenceof supported catalysts comprising Cu(I) ions. The 4-vinylcyclohexeneformed can be dehydrogenated to ethylbenzene in a downstreamdehydrogenation step or oxydehydrogenated directly to styrene in thepresence of oxygen.

[0003] U.S. Pat. No. 5,196,621 discloses a process for the dimerizationof butadiene in the liquid phase over aluminosilicates impregnated withCu(I) ions as dimerization catalysts, preferably over zeolites such asfaujasite, mordenite, zeolite L, omega zeolite and beta zeolite whichhave been impregnated with Cu(I) ions. Furthermore, clay minerals suchas montmorillonite containing Cu(I) ions and Cu(I)-containingnonzeolitic amorphous aluminum oxide/silicon dioxide mixtures, silicondioxide or aluminum oxide are also mentioned as suitable catalysts.

[0004] Butadiene is prepared mainly by thermal cracking of saturatedhydrocarbons, usually using naphtha as raw material. The cracking ofnaphtha gives a hydrocarbon mixture comprising methane, ethane, ethene,acetylene, propane, propene, propyne, allene, butenes, butadiene,butynes, methylallene, C₅-hydrocarbons and higher hydrocarbons.Acetylenically unsaturated hydrocarbons, in particular, in the crackinggas, e.g. acetylene, propyne, 1-butyne, 2-butyne, butenyne anddiacetylene interfere in the dimerization. Even traces of thesecompounds can poison the copper-containing dimerization catalyst.Butynes and allenes likewise react with butadiene in a Diels-Alderreaction and lead to by-product formation. Particular problems arepresented by the butynes which are very difficult to separate frombutadiene by distillation or extraction. When using butadiene fromcrackers, it is therefore necessary to precede the butadienedimerization by a hydrogenation step in which the butynes areselectively partially hydrogenated to the corresponding butenes. Inother uses of butadiene, too, triply unsaturated C₄-hydrocarbonsgenerally interfere.

[0005] A further disadvantage is that the cracking of naphtha or otherhydrocarbon mixtures produces a complex hydrocarbon mixture. Thus, theproduction of butadiene in a cracking process inevitably leads toformation of relatively large amounts of ethene or propene ascoproducts.

[0006] It is an object of the present invention to provide an economicalprocess for preparing 4-vinylcyclohexene, ethylbenzene or styrene inwhich coproducts are formed to a lesser extent. A particular object ofthe invention is to provide a preparation of 4-vinylcyclohexene,ethylbenzene and styrene which has a new raw materials basis.

[0007] We have found that this object is achieved by a process forpreparing 4-vinylcyclohexene, which comprises the steps

[0008] (A) providing an n-butane-containing feed gas stream,

[0009] (B) feeding the n-butane-containing feed gas stream into at leastone dehydrogenation zone and dehydrogenating n-butane to butadiene togive a product stream comprising butadiene, n-butane, possibly 1-buteneand 2-butene and possibly water vapor and other secondary constituents,

[0010] (C) feeding the product stream from dehydrogenation, ifappropriate after separating off water vapor and secondary constituents,into a dimerization zone and catalytically dimerizing butadiene to givea product stream comprising 4-vinylcyclohexene, n-butane and possibly1-butene, 2-butene and unreacted butadiene, and

[0011] (D) separating off 4-vinylcyclohexene from the product streamfrom the dimerization and recirculating n-butane and possibly 1-butene,2-butene and unreacted butadiene to the dehydrogenation zone.

[0012] No significant amounts of acetylenically unsaturated hydrocarbonsor allenes are formed as by-products in the dehydrogenation of n-butane.Thus, a partial hydrogenation of the gas mixture used in thedimerization of butadiene can be omitted.

[0013] In a first process stage A, an n-butane-containing feed gasstream is provided. n-Butane-rich gas mixtures such as liquefiedpetroleum gas (LPG) are usually used as raw materials for this purpose.LPG consists essentially of C₂-C₅-hydrocarbons. The composition of LPGcan fluctuate widely. The LPG used advantageously contains at least 10%by weight of butanes.

[0014] In one variant of the process of the present invention, theprovision of the n-butane-containing dehydrogenation feed streamcomprises the steps

[0015] (A1) providing a liquefied petroleum gas (LPG) stream,

[0016] (A2) separating off propane and, if appropriate, methane, ethaneand pentanes from the LPG stream to give a stream comprising butanes,

[0017] (A3) separating off isobutane from the stream comprising butanesto give the n-butane-containing feed gas stream and, if desired,isomerizing the isobutane which has been separated off to give ann-butane/isobutane mixture and recirculating the n-butane/isobutanemixture to the isobutane separation step.

[0018] Propane and, if appropriate, methane, ethane and pentanes areseparated off in one or more customary rectification columns. Forexample, low boilers (methane, ethane, propane) can be separated off viathe top in a first column and high boilers (pentanes) can be separatedoff at the bottom in a second column. This gives a stream comprisingbutanes (n-butane and isobutane) from which isobutane is separated offin a, for example, customary rectification column. The remainingn-butane-containing stream is used as feed gas stream for the subsequentdehydrogenation of butane.

[0019] The isobutane stream which has been separated off is preferablysubjected to isomerization. For this purpose, the isobutane-containingstream is fed into an isomerization reactor. The isomerization ofisobutane to n-butane can be carried out as described in GB-A 2 018 815.This gives an n-butane/isobutane mixture which is fed into then-butane/isobutane separation column.

[0020] In a process stage (B), n-butane is dehydrogenated to butadiene.

[0021] In an embodiment of the process of the invention, the butadienedehydrogenation is carried out as a nonoxidative catalyticdehydrogenation. In this, n-butane is partially dehydrogenated over adehydrogenation-active catalyst in a dehydrogenation reactor to givebutadiene. In addition, 1-butene and 2-butene are formed from n-butane.The dehydrogenation also results in the formation of hydrogen and smallamounts of methane, ethane, ethene, propane and propene. Depending onthe way in which the dehydrogenation is carried out, carbon oxides (CO,CO₂), water and nitrogen can also be present in the product gas mixturefrom the butane dehydrogenation. In addition, unreacted butane ispresent in the product gas mixture.

[0022] The nonoxidative catalytic dehydrogenation of butane can becarried out with or without the use of an oxygen-containing gas ascofeed.

[0023] The nonoxidative catalytic dehydrogenation of n-butane can inprinciple be carried out in all types of reactor known from the priorart and by all known modes of operation. A comparatively comprehensivedescription of dehydrogenation processes which are suitable for thepurposes of the present invention is given in “Catalytica® StudiesDivision, Oxidative Dehydrogenation and Alternative DehydrogenationProcesses” (Study Number 4192 OD, 1993, 430 Ferguson Drive, MountainView, Calif., 94043-5272, USA).

[0024] A suitable type of reactor is a fixed-bed tube or shell-and-tubereactor. In these, the catalyst (dehydrogenation catalyst and, whenusing oxygen as cofeed, possibly a specific oxidation catalyst) ispresent as a fixed bed in a reaction tube or in a bundle of reactiontubes. The reaction tubes are usually heated indirectly by a gas, e.g. ahydrocarbon such as methane, being burnt in the space surrounding thereaction tubes. It is advantageous to employ this indirect form ofheating only over the first about 20-30% of the length of the fixed bedand to heat the remaining length of the bed to the required reactiontemperature by means of the radiative heat produced as a result of theindirect heating. Customary internal diameters of the reaction tube(s)are from about 10 to 15 cm. A typical shell-and-tube dehydrogenationreactor comprises from about 300 to 1 000 reaction tubes. Thetemperature in the interior of the reaction tubes is usually in therange from 300 to 1 200° C., preferably in the range from 500 to 1 000°C. The working pressure is usually in the range from 0.5 to 8 bar,frequently in the range from 1 to 2 bar, when using a small degree ofsteam dilution (as in the Linde process for propane dehydrogenation),but may also be in the range from 3 to 8 bar when using a high degree ofsteam dilution (as in the “steam active reforming process” (STARprocess) of Phillips Petroleum Co. for the dehydrogenation of propane orbutane, cf. U.S. Pat. No. 4,902,849, U.S. Pat. No. 4,996,387 and U.S.Pat. No. 5,389,342). Typical space velocities over the catalyst (GSHV)are from 500 to 2 000 h⁻¹, based on the hydrocarbon used. The catalystgeometry can be, for example, spherical or cylindrical (hollow orsolid).

[0025] The nonoxidative catalytic dehydrogenation of n-butane can alsobe carried out in the presence of a heterogeneous catalyst in afluidized bed, as described in Chem. Eng. Sci. 1992 b, 47 (9-11) 2313.It is advantageous to operate two fluidized beds in parallel, with onegenerally being in the regeneration mode. The working pressure istypically from 1 to 2 bar, and the dehydrogenation temperature isgenerally from 550 to 600° C. The heat required for the dehydrogenationis introduced into the reaction system by preheating the dehydrogenationcatalyst to the reaction temperature. Mixing in an oxygen-containingcofeed can enable the preheater to be omitted and the required heat tobe generated directly in the reactor system by combustion of hydrogen inthe presence of oxygen. In addition, a hydrogen-containing cofeed canalso be mixed in if appropriate.

[0026] The nonoxidative catalytic dehydrogenation of n-butane can becarried out with or without use of oxygen-containing gas as cofeed in atray reactor. This contains one or more successive catalyst beds. Thenumber of catalyst beds can be from 1 to 20, advantageously from 1 to 6,preferably from 1 to 4 and in particular from 1 to 3. The reaction gaspreferably flows radially or axially through the catalyst beds. Ingeneral, such a tray reactor is operated using a fixed catalyst bed. Inthe simplest case, the fixed catalyst beds are installed axially or inthe annular gaps of concentrically arranged cylindrical gratings in ashaft furnace reactor. One shaft furnace reactor corresponds to onetray. Carrying out the dehydrogenation in a single shaft furnace reactorcorresponds to a preferred embodiment, with an oxygen-containing cofeedbeing able to be employed. In a further preferred embodiment, thedehydrogenation is carried out in a tray reactor having 3 catalyst beds.When the reactor is operated without oxygen-containing gas as cofeed,the reaction gas mixture is subjected to intermediate heating on its wayfrom one catalyst bed to the next catalyst bed in the tray reactor, e.g.by passing it over heat exchanger surfaces heated by means of hot gasesor by passing it through tubes heated by hot combustion gases.

[0027] In a preferred embodiment of the process of the presentinvention, the nonoxidative catalytic dehydrogenation of n-butane iscarried out autothermally. For this purpose, additional oxygen is mixedinto the reaction gas mixture of the n-butane dehydrogenation in atleast one reaction zone and the hydrogen and/or hydrocarbons present inthe reaction gas mixture is at least partially burnt so as to generateat least part of the required heat of dehydrogenation directly in thereaction gas mixture in the reaction zone or zones. A feature ofautothermal operation compared to oxidative operation is, for example,the presence of hydrogen in the exit gas. In oxidative processes, nosignificant amounts of free hydrogen are formed.

[0028] In general, the amount of oxygen-containing gas added to thereaction gas mixture is chosen so that combustion of the hydrogenpresent in the reaction gas mixture and any hydrocarbons present in thereaction gas mixture and/or carbon present in the form of carbondeposits generates the heat necessary for the dehydrogenation of then-butane. In general, the total amount of added oxygen is, based on thetotal amount of butane, from 0.001 to 0.5 mol/mol, preferably from 0.005to 0.2 mol/mol, particularly preferably from 0.05 to 0.2 mol/mol. Oxygencan be used either as pure oxygen or in admixture with inert gases as anoxygen-containing gas, for example in the form of air. The inert gasesand the resulting combustion gases generally have an additional dilutingeffect and thus aid the heterogeneously catalyzed dehydrogenation.

[0029] The hydrogen burned to generate heat is the hydrogen formed inthe catalytic dehydrogenation of n-butane and also any additionalhydrogen added as hydrogen-containing gas to the reaction gas mixture.Preference is given to adding such an amount of hydrogen that the molarratio H₂/O₂ in the reaction gas mixture directly after the introductionof oxygen is from 1 to 10 mol/mol, preferably from 2 to 5 mol/mol. Inthe case of multistage reactors, this applies to each intermediateaddition of oxygen-containing and, if applicable, hydrogen-containinggas.

[0030] The combustion of hydrogen occurs catalytically. Thedehydrogenation catalyst used generally also catalyzes the combustion ofhydrocarbons and of hydrogen with oxygen, so that in principle nospecific oxidation catalyst other than this is necessary. In oneembodiment, the dehydrogenation process is carried out in the presenceof one or more oxidation catalysts which selectively catalyze thecombustion of hydrogen to oxygen in the presence of hydrocarbons. As aresult, the combustion of these hydrocarbons with oxygen to form CO, CO₂and water occurs to only a minor extent. The dehydrogenation catalystand the oxidation catalyst are preferably present in different reactionzones.

[0031] In the case of a multistage reaction, the oxidation catalyst canbe present in only one reaction zone, in a plurality of reaction zonesor in all reaction zones.

[0032] The catalyst which selectively catalyzes the oxidation ofhydrogen is preferably located at places in which the oxygen partialpressure is higher than at other places in the reactor, in particular inthe vicinity of the feed point for the oxygen-containing gas. Theintroduction of oxygen-containing gas and/or hydrogen-containing gas canbe carried out at one or more points in the reactor.

[0033] In one embodiment of the process of the present invention, anintermediate addition of oxygen-containing gas and ofhydrogen-containing gas is carried out upstream of each tray of a trayreactor. In a further embodiment of the process of the presentinvention, the introduction of oxygen-containing gas and ofhydrogen-containing gas is carried out upstream of each tray apart fromthe first tray. In one embodiment, a bed of a specific oxidationcatalyst is present downstream of each addition point, followed by a bedof the dehydrogenation catalyst. In a further embodiment, no specificoxidation catalyst is present. The dehydrogenation temperature isgenerally from 400 to 1 100° C., and the pressure in the last catalystbed of the tray reactor is generally from 0.2 to 5 bar, preferably from1 to 3 bar. The space velocity (GSHV) is generally from 500 to 2 000h⁻¹, and in a high-load process even up to 100 000 h⁻¹, preferably from4 000 to 16 000 h⁻¹.

[0034] A preferred catalyst which selectively catalyzes the combustionof hydrogen comprises oxides and/or phosphates selected from the groupconsisting of the oxides and phosphates of germanium, tin, lead,arsenic, antimony or bismuth. A further preferred catalyst whichcatalyzes the combustion of hydrogen comprises a noble metal oftransition group VIII and/or I.

[0035] The dehydrogenation catalysts used generally comprise a supportand an active composition. The support usually comprises a thermallystable oxide or mixed oxide. The dehydrogenation catalysts preferablycomprise a metal oxide which is selected from the group consisting ofzirconium dioxide, zinc oxide, aluminum oxide, silicon dioxide, titaniumdioxide, magnesium oxide, lanthanum oxide, cerium oxide and mixturesthereof as support. The mixtures can be physical mixtures or chemicalmixed phases such as magnesium- or zinc-aluminum mixed oxides. Preferredsupports are zirconium dioxide and/or silicon dioxide, particularlypreferably mixtures of zirconium dioxide and silicon dioxide.

[0036] The active composition of the dehydrogenation catalysts generallycomprises one or more elements of transition group VIII, preferablyplatinum and/or palladium, particularly preferably platinum. Thedehydrogenation catalysts can further comprise one or more elements ofmain groups I and/or II, preferably potassium and/or cesium.Furthermore, the dehydrogenation catalysts may also comprise one or moreelements of transition group III including the lanthanides andactinides, preferably lanthanum and/or cerium. Finally, thedehydrogenation catalysts may comprise one or more elements of maingroups III and/or IV, preferably one or more elements from the groupconsisting of boron, gallium, silicon, germanium, tin and lead,particularly preferably tin.

[0037] In a preferred embodiment, the dehydrogenation catalyst comprisesat least one element of transition group VIII, at least one element ofmain groups I and/or II, at least one element of main groups III and/orIV and at least one element of transition group III including thelanthanides and actinides.

[0038] For the purposes of the present invention, it is possible to use,for example, all dehydrogenation catalysts disclosed in WO 99/46039,U.S. Pat. No. 4,788,371, EP-A 705 136, WO 99/29420, U.S. Pat. No.5,220,091, U.S. Pat. No. 5,430,220, U.S. Pat. No. 5,877,369, EP 0 117146, DE-A 199 37 106, DE-A 199 37 105 and DE-A 199 37 107.

[0039] Particularly preferred catalysts for the above-described variantsof the autothermal dehydrogenation of butane are the catalysts describedin examples 1, 2, 3 and 4 of DE-A 199 37 107.

[0040] The n-butane dehydrogenation is preferably carried out in thepresence of steam. The added steam serves as heat carrier and aids thegasification of organic deposits on the catalysts, thus counteringcarbonization of the catalysts and increasing the operating life of thecatalyst. The organic deposits are in this way converted into carbonmonoxide and carbon dioxide and possibly water.

[0041] The dehydrogenation catalyst can be regenerated in a manner knownper se. Thus, steam can be added to the reaction gas mixture or anoxygen-containing gas can be passed over the catalyst bed at elevatedtemperature from time to time so that the deposited carbon is burnedoff. The presence of steam acting as diluent also has a positive effecton the position of the chemical equilibrium, which is shifted to theside of the dehydrogenation products. If desired, a reduction using ahydrogen-containing gas may be carried out after the regeneration bymeans of steam.

[0042] The butane dehydrogenation gives a gas mixture comprisingbutadiene, 1-butene, 2-butene and unreacted n-butane together withsecondary constituents. Usual secondary constituents are hydrogen, watervapor, nitrogen, CO and CO₂, methane, ethane, ethene, propane andpropene. The composition of the gas mixture leaving the dehydrogenationstage can vary greatly as a function of the way in which thedehydrogenation is carried out. Thus, the preferred autothermaldehydrogenation with addition of oxygen and further hydrogen gives aproduct gas mixture having a comparatively high content of water vaporand carbon oxides. When no oxygen is added, the product gas mixture fromthe nonoxidative dehydrogenation has a comparatively high hydrogencontent.

[0043] The product gas stream from the nonoxidative autothermaldehydrogenation of butane typically comprises from 0.1 to 15% by volumeof butadiene, from 1 to 15% by volume of 1-butene, from 1 to 20% byvolume of 2-butene, from 20 to 70% by volume of butane, from 5 to 70% byvolume of water vapor, from 0 to 5% by volume of low-boilinghydrocarbons (methane, ethane, ethene, propane and propene), from 0 to30% by volume of hydrogen, from 0 to 30% by volume of nitrogen and from0 to 5% by volume of carbon oxide.

[0044] In a preferred embodiment, the nonoxidative catalyticdehydrogenation is followed by an oxidative dehydrogenation.

[0045] The dehydrogenation of n-butane to butadiene comprises, in thispreferred embodiment, the steps

[0046] (B1) feeding the n-butane-containing feed gas stream into a firstdehydrogenation zone and catalytically, nonoxidatively dehydrogenatingn-butane to 1-butene, 2-butene and possibly butadiene to give a productgas stream comprising n-butane, 1-butene, 2-butene, possibly butadieneand possibly secondary constituents,

[0047] (B2) feeding the product gas stream comprising n-butane,1-butene, 2-butene, possibly butadiene and possibly secondaryconstituents into a second dehydrogenation zone and oxidativelydehydrogenating 1-butene and 2-butene to butadiene to give a product gasstream comprising butadiene, n-butane, water vapor and possiblysecondary constituents.

[0048] The catalytic nonoxidative dehydrogenation (B1) of n-butane to1-butene, 2-butene and possibly butadiene is preferably carried out asdescribed above as an autothermal dehydrogenation.

[0049] The oxydehydrogenation (B2) can in principle be carried out usingall types of reactor and modes of operation known from the prior art,for example in a fluidized bed, in a tray furnace or in a fixed-bed tubeor shell-and-tube reactor. Preference is given to using the latter inthe process of the present invention. Carrying out the oxidativedehydrogenation requires a gas mixture which has a molar oxygen:n-buteneratio of at least 0.5. Preference is given to an oxygen:n-butene ratioof from 0.55 to 50. To set this value, the product gas mixture from thecatalytic dehydrogenation is generally mixed with oxygen or anoxygen-containing gas, for example air. The oxygen-containing gasmixture obtained is then fed to the oxydehydrogenation.

[0050] Catalysts which are particularly useful for the oxidativedehydrogenation (oxydehydrogenation) of the n-butenes to 1,3-butadieneare generally based on an Mo—Bi—O-containing multimetal oxide system,which generally further comprises iron. In general, the catalyst systemfurther comprises additional components from groups 1 to 15 of thePeriodic Table, for example potassium, magnesium, zirconium, chromium,nickel, cobalt, cadmium, tin, lead, germanium, lanthanum, manganese,tungsten, phosphorus, cerium, aluminum or silicon.

[0051] Suitable catalysts and their preparation are described, forexample, in U.S. Pat. No. 4,423,281 (Mo₁₂BiNi₈Pb_(0.5)Cr₃K_(0.2)O_(x)and Mo₁₂Bi_(b)Ni₇Al₃Cr_(0.5)K_(0.5)O_(x)), U.S. Pat. No. 4,336,409(Mo₁₂BiNi₆Cd₂Cr₃P_(0.5)O_(x)), DE-A 26 00 128(Mo₁₂BiNi_(0.5)Cr₃P_(0.5)Mg_(7.5)K_(0.1)O_(x)+Si₂) and DE-A 24 40 329(Mo₁₂BiCo_(4.5)Ni_(2.5)Cr₃P_(0.5)K_(0.1)O_(x)), which are herebyexplicitly incorporated by reference.

[0052] The stoichiometry of the active composition of many of themultimetal oxide catalysts suitable for the oxydehydrogenation of then-butenes to 1,3-butadiene have the formula (I)

Mo_(0.2)Bi_(a)Fe_(b)Co_(c)Ni_(d)Cr_(e)X¹ _(f)K_(g)O_(x)   (I),

[0053] where the variables have the following meanings:

[0054] X¹=W, Xn, Mn, La, Ce, Ge, Ti, Zr, Hf, Nb, P, Si, Sb, Al, Cdand/or Mg;

[0055] a=0.5 to 5, preferably from 0.5 to 2;

[0056] b=0 to 5, preferably from 2 to 4;

[0057] c=0 to 10, preferably from 3 to 10;

[0058] d=0 to 10;

[0059] e=0 to 10, preferably from 0.1 to 4;

[0060] f=0 to 5, preferably from 0.1 to 2;

[0061] g=0 to 2, preferably from 0.01 to 1; and

[0062] x=a number which is determined by the valence and abundance ofthe elements other than oxygen in (I).

[0063] In the process of the present invention, preference is given tousing an Mo—Bi—Fe—O-containing multimetal oxide system for theoxydehydrogenation, with particular preference being given to anMo—Bi—Fe—Cr—O— or Mo—Bi—Fe—Zr—O— containing multimetal oxide system.Preferred systems are described, for example, in U.S. Pat. No. 4,547,615(Mo₁₂BiFe_(0.1)Ni₈ZrCr₃K_(0.2)O_(x) andMo₁₂BiFe_(0.1)Ni₈AlCr₃K_(0.2)O_(x)), U.S. Pat. No. 4,424,141(Mo₁₂BiFe₃Co_(4.5)Ni_(2.5)P_(0.5)K_(0.1)O_(x)+SiO₂), DE-A 25 30 959(Mo₁₂BiFe₃Co_(4.5)Ni_(2.5)Cr_(0.5)K_(0.1)O_(x),Mo_(13.75)BiFe₃Co_(4.5)Ni_(2.5)Ge_(0.5)K_(0.8)O_(x),Mo₁₂BiFe₃Co_(4.5)Ni_(2.5)Mn_(0.5)K_(0.1)O_(x) andMo₁₂BiFe₃Co_(4.5)Ni_(2.5)La_(0.5)K_(0.1)O_(x)), U.S. Pat. No. 3,911,039Mo₁₂BiFe₃Co_(4.5)Ni_(2.5)Sn_(0.5)K_(0.1)O_(x)), DE-A-25 30 959 andDE-A-24 47 825 (Mo₁₂BiFe₃Co_(4.5)Ni_(2.5)W_(0.5)K_(0.1)O_(x)). Thepreparation and characterization of the abovementioned catalysts arecomprehensively described in the documents cited, which are herebyexplicitly incorporated by reference.

[0064] The catalyst for the oxydehydrogenation is generally used asshaped bodies having a mean size of above 2 mm. Due to the need to payattention to the pressure drop during operation of the process,relatively small shaped bodies are generally unsuitable. Suitable shapedbodies which may be mentioned are, for example, pellets, cylinders,hollow cylinders, rings, spheres, rods, wagon wheels or extrudates.Special shapes such as “trilobes” and “tristars” (cf. EP-A-0 593 646) orshaped bodies having at least one recess on the outside (cf. U.S. Pat.No. 5,168,090) are likewise possible.

[0065] In general, the catalyst used can be employed as an all-activecatalyst. In this case, the entire shaped catalyst body consists of theactive composition, including any auxiliaries, e.g. graphite or poreformers, and further components. In particular, it has been found to beadvantageous to use the Mo—Bi—Fe—O-containing catalyst which ispreferably used for the oxydehydrogenation of the n-butenes to butadieneas an all-active catalyst. It is also possible to apply the activecompositions of the catalysts to a support, for example an inorganic,oxidic shaped body. Such catalysts are generally referred to as coatedcatalysts.

[0066] The oxydehydrogenation of the n-butenes to butadiene is generallycarried out at from 220 to 490° C., preferably 250 to 450° C. Forpractical reasons, it is usual to select a reactor inlet pressure whichis sufficient to overcome the flow resistances present in the plant andthe subsequent work-up. This reactor inlet pressure is generally from0.005 to 1 MPa gauge pressure, preferably from 0.01 to 0.5 MPa gaugepressure. The gas pressure employed in the inlet region of the reactornaturally decreases over the overall bed of catalysts and inertcomponents.

[0067] Coupling of the nonoxidative catalytic, preferably autothermal,dehydrogenation with the oxidative dehydrogenation of the n-butenesformed gives a very much higher yield of butadiene based on n-butaneused. Furthermore, the nonoxidative dehydrogenation can be carried outunder mild conditions. Comparable yields of butadiene would be able tobe achieved by means of an exclusively nonoxidative dehydrogenation onlyat the expense of significantly reduced selectivities.

[0068] The product gas stream leaving the oxidative dehydrogenationcomprises butadiene and unreacted n-butane together with water vapor. Assecondary constituents, it generally further comprises carbon monoxide,carbon dioxide, nitrogen, oxygen, methane, ethane, ethene, propane andpropene, possibly hydrogen and oxygen-containing hydrocarbons (organicoxygen compounds). It generally contains only small proportions of1-butene and 2-butene.

[0069] The product gas stream leaving the oxidative dehydrogenation cancomprise, for example, from 1 to 20% by volume of butadiene, from 0 to1% by volume of 1-butene, from 0 to 1% by volume of 2-butene, from 0 to50% by volume of n-butane, from 2 to 50% by volume of water vapor, from0 to 5% by volume of low-boiling hydrocarbons (methane, ethane, ethene,propane and propene), from 0 to 20% by volume of hydrogen, from 0 to 90%by volume of nitrogen, from 0 to 5% by volume of carbon oxides and from0 to 3% by weight of organic oxygen compounds.

[0070] After leaving the dehydrogenation stage(s), the hot gas mixture,which is generally at a temperature of from 500 to 650° C. when thedehydrogenation is carried out entirely autothermally and generally atfrom 220 to 490° C. when the autothermal dehydrogenation is followed byan oxidative dehydrogenation, is usually cooled by means of water. Thisresults in water vapor and any high-boiling organic secondaryconstituents being condensed out. The low-boiling secondary constituentssuch as hydrogen, carbon monoxide, carbon dioxide, nitrogen, methane,ethane, ethene, propane and propene which are present in thedehydrogenation gas mixture in addition to butadiene, n-butane andpossibly 1-butene and 2-butene are usually separated off from theC₄-hydrocarbons prior to the butadiene dimerization.

[0071] The low-boiling secondary constituents can be separated off bycustomary rectification methods.

[0072] The low-boiling secondary constituents can also be separated offby means of a high-boiling absorption medium in an absorption/desorptioncycle. This separates essentially all low-boiling secondary constituents(nitrogen, argon, hydrogen, methane, ethane, ethene, propane, propene,carbon oxides, oxygen) from the product gas stream from thedehydrogenation of n-butane.

[0073] For this purpose, the C₄-hydrocarbons are absorbed in an inertabsorption medium in an absorption stage so as to give an absorptionmedium laden with the C₄-hydrocarbons and an off-gas comprising theother secondary constituents. In a desorption stage, the C₄-hydrocarbonsand traces of secondary constituents are liberated again from theabsorption medium.

[0074] Inert absorption media used in the absorption stage are generallyhigh-boiling nonpolar solvents in which the hydrocarbon mixture to beseparated off has a significantly higher solubility than do the otherconstituents of the product gas mixture. The absorption can be carriedout by simply passing the product gas mixture through the absorptionmedium. However, it can also be carried out in columns or in rotaryabsorbers. Absorption can be carried out in cocurrent, countercurrent orin the crossflow mode. Suitable absorption columns are, for example,tray columns having bubble cap trays, centrifugal trays and/or sievetrays, columns containing structured packing, e.g. sheet metal packinghaving a specific surface area of from 100 to 1 000 m²/m³, e.g.Mellapak® 250 Y, and columns packed with random packing elements.However, trickle and spray towers, graphite block absorbers, surfaceabsorbers such as thick film absorbers and thin film absorbers and alsorotary columns, plate scrubbers, cross-spray scrubbers and rotaryscrubbers are also possible.

[0075] Suitable absorption media are comparatively nonpolar organicsolvents, for example aliphatic C₈-C₁₈-alkenes, or aromatic hydrocarbonssuch as middle oil fractions from paraffin distillation or ethers havingbulky groups, or mixtures of these solvents. A polar solvent such asdimethyl phthalate may also be added to these. Suitable absorption mediaalso include esters of benzoic acid and phthalic acid withstraight-chain C₁-C₈-alkanols, e.g. n-butyl benzoate, methyl benzoate,ethyl benzoate, dimethyl phthalate, diethyl phthalate, and also heattransfer fluids such as biphenyl and diphenyl ether, their chlorinederivatives and also triarylalkenes. One suitable absorption medium is amixture of biphenyl and diphenyl ether, preferably in the azeotropiccomposition, for example the commercially available Diphyl®. Thissolvent mixture frequently contains from 0.1 to 25% by weight ofdimethyl phthalate. Further suitable absorption media are octanes,nonanes, decanes, undecanes, dodecanes, tridecanes, tetradecanes,pentadecanes, hexadecanes, heptadecanes and octadecanes and fractionscomprising predominantly these linear alkanes obtained from refinerystreams.

[0076] For desorption, the laden absorption medium is heated and/ordepressurized to a lower pressure. Alternatively, desorption can also becarried out by stripping or by a combination of depressurization,heating and stripping in one or more process steps. The absorptionmedium which has been regenerated in the desorption stage is returned tothe absorption stage.

[0077] Organic oxygen compounds can be separated off from the remainingC₄-hydrocarbon-containing stream in a further separation step, which canlikewise be configured as an absorption/desorption cycle or as arectification. Organic oxygen compounds are, for example, furan andmaleic anhydride.

[0078] The remaining stream, which consists essentially of butadiene,n-butane, 1-butene and 2-butene, is fed to the dimerization.

[0079] In a subsequent process stage (C), butadiene is dimerizedcatalytically to form 4-vinylcyclohexene.

[0080] The dimerization of butadiene can be carried out in the liquidphase over a copper-containing catalyst. Suitable dimerization catalystsare aluminosilicates impregnated with Cu(I) ions, for example zeolitessuch as faujasite, mordenite, zeolite L, omega zeolite or beta zeolitewhich have been impregnated with Cu(I) ions, as described in U.S. Pat.No. 5,196,621. Further suitable supports are clay minerals such asmontmorillonite, nonzeolitic amorphous aluminum oxide/silicon dioxidemixtures, silicon dioxide or aluminum oxide.

[0081] The dimerization of butadiene can be carried out in all customaryreaction apparatuses in a fixed-bed or suspension mode, for example intube reactors, continuously operated stirred vessels or cascades ofstirred vessels. The reaction temperature is typically from 70 to 170°C., preferably from 100 to 130° C., and the reaction pressure is from 7to 70 bar, preferably from 20 to 35 bar.

[0082] 4-Vinylcyclohexene is formed high selectively in the dimerizationof butadiene. 1-Butene and 2-butene but also possible traces of propenegenerally do not react under the reaction conditions since they do nothave a double bond which is activated for the Diels-Alder reaction.

[0083] The dimerization of butadiene can also be carried out, asdescribed in EP-A 0 397 266, in the liquid phase in a suitable solventusing iron nitrosyl chloride, cobalt nitrosyl chloride or nickelnitrosyl chloride in the presence of carbon monoxide and tin, zinc,manganese and/or magnesium. Examples of suitable solvents are ethyleneglycol dialkyl ethers or diethylene glycol dialkyl ethers,tetrahydrofuran and acetonitrile. The reaction temperature is generallyfrom 20 to 175° C., and the reaction pressure is from 1 to 70 bar. Thevinylcyclohexene formed can subsequently be separated from the solventby distillation.

[0084] In a further process stage (D), 4-vinylcyclohexene is separatedoff from the product stream from the butadiene dimerization. Theseparation can be carried out in a customary rectification column. Thisgenerally gives a stream of crude 4-vinylcyclohexene which may containsmall amounts of C₈-by-products and a C₄ stream comprising n-butane,1-butene, 2-butene and unreacted butadiene. The C₄ stream can berecirculated to the n-butane dehydrogenation.

[0085] The 4-vinylcyclohexene obtained can, if appropriate after priorpurification, subsequently be dehydrogenated to ethylbenzene or elseoxydehydrogenated to styrene in the presence of oxygen.

[0086] The present invention therefore also provides a process forpreparing ethyl benzene or styrene comprising the steps (A), (B), (C)and (D) as described above and the additional step

[0087] (E) feeding 4-vinylcyclohexene into a further dehydrogenationzone and catalytically dehydrogenating it to ethylbenzene oroxydehydrogenating it in the presence of oxygen to give styrene.

[0088] The dehydrogenation of 4-vinylcyclohexene to ethylbenzene can, asdescribed in WO 94/01385, be carried out in the gas phase over magnesiumoxide, zinc oxide, calcium oxide, strontium oxide or barium oxide ascatalyst. The dehydrogenation can be carried out in many suitablereactors, e.g. continuously operated fixed-bed or fluidized-bedreactors. The reaction temperature is generally from 400 to 625° C.,preferably from 450 to 600° C., and the reaction pressure is generallyfrom 1 to 25 bar, preferably from 1 to 10 bar.

[0089] The dehydrogenation of 4-vinylcyclohexene to ethylbenzene canalso, as described in U.S. Pat. No. 3,903,185, be carried out at from350 to 450° C. and a pressure of from 2.5 to 30 bar in the presence ofhydrogen over a catalyst comprising one or more elements of transitiongroups VI to VIII or oxides thereof on an aluminum oxide support.Preferred catalysts are rhenium, palladium and/or platinum on aluminumoxide and cobalt oxide/molybdenum oxide on aluminum oxide.

[0090] The dehydrogenation of 4-vinylcyclohexene to ethylbenzene canalso, as described in U.S. Pat. No. 4,029,715, be carried out at 400° C.and atmospheric pressure in the presence of an inert gas such as steamor nitrogen over cobalt molybdate/potassium oxide on aluminum oxide ascatalyst.

[0091] Furthermore, the dehydrogenation of 4-vinylcyclohexene toethylbenzene can be carried out in the gas phase at atmospheric pressureand temperatures of up to 300° C. over palladium on magnesium oxide ascatalyst.

[0092] The dehydrogenation gives a crude ethylbenzene which comprises,as secondary constituents, unreacted 4-vinylcyclohexane andethylcyclohexane as by-product.

[0093] The 4-vinylcyclohexene formed in the dimerization of butadienecan also be dehydrogenated in the presence of oxygen so as to convert itdirectly into styrene. Appropriate processes are described, for example,in U.S. Pat. No. 3,502,736 and DE-A 2 612 082.

[0094] In a modification of the above-described process, thedehydrogenation of 4-vinylcyclohexene to styrene is carried out togetherwith the n-butane dehydrogenation. In this way, the separation of4-vinylcyclohexene from the product stream from the dimerization can bedispensed with.

[0095] This comprises the steps

[0096] (A) providing an n-butane-containing feed gas stream,

[0097] (B′) feeding the n-butane-containing feed gas stream and a4-vinylcyclohexene-containing gas stream into a dehydrogenation zone andjointly dehydrogenating n-butane and 4-vinylcyclohexene in the presenceof oxygen to give a product stream comprising styrene, butadiene,n-butane, 1-butene, 2-butene, possibly ethylbenzene and furthersecondary constituents,

[0098] (C′) separating off styrene and, if applicable, ethylbenzene andfurther high-boiling secondary constituents from the product stream fromthe dehydrogenation,

[0099] (D′) feeding the stream comprising butadiene, n-butane, 1-buteneand 2-butene into a dimerization zone and catalytically dimerizingbutadiene to give a product stream comprising 4-vinylcyclohexene,n-butane, 1-butene, 2-butene and possibly unreacted butadiene,

[0100] (E′) isolating the 4-vinylcyclohexene-containing gas stream fromthe product stream from the dimerization and feeding it into thedehydrogenation zone.

[0101] Further high-boiling secondary constituents which can be formedin the dehydrogenation and are separated off together with styrene and,if applicable, ethylbenzene are xylenes, toluene and benzene.

[0102] Suitable catalysts for the joint dehydrogenation of n-butane and4-vinylcyclohexane in the presence of oxygen are those of theabove-described dehydrogenation catalysts which comprise a noble metalof transition group VIII, preferably platinum and/or palladium, on asupport. In addition, the dehydrogenation catalysts may comprise one ormore elements of main groups I and/or II, preferably potassium and/orcesium, one or more elements of transition group III including thelanthanides and actinides, preferably lanthanum and/or cerium, one ormore elements of main groups III and/or W, preferably boron, gallium,silicon, germanium, tin and/or lead, particularly preferably tin.

[0103] Preferred embodiments of the process of the present invention areillustrated below with reference to the drawings.

[0104]FIG. 1 shows the process flow diagram of a preferred embodiment ofthe process of the present invention. A feed stream 1 composed ofliquefied petroleum gas (LPG), which consists essentially of propane,n-butane and isobutane, is fed into a rectification column 2 andseparated into a stream 3 consisting essentially of propane and possiblymethane and ethane and a stream 4 consisting essentially of n-butane andisobutane. In the rectification column 5, the butane mixture isseparated into isobutane 6 and n-butane 9, with isobutane beingisomerized in the isomerization reactor 7 to give an n-butane/isobutanemixture 8 which is fed back into the rectification column 5. n-Butane isintroduced as feed gas stream 9 into the dehydrogenation reactor 11which is preferably operated under autothermal conditions with additionof oxygen or air as cofeed 10. The product gas stream 12 leaving thedehydrogenation reactor, which comprises butadiene, 1-butene, 2-buteneand unreacted n-butane together with by-products such as hydrogen,carbon oxides, nitrogen, water vapor, methane, ethane, ethene, propaneand/or propene, is, after precooling in heat exchangers, cooled in thecooling and condensation unit 13, for example a bed through which watertrickles or a falling film condenser, to such an extent that water andhigh-boiling organic by-products are condensed out and are dischargedfrom the process as stream 14. The product of gas constituents whichhave not condensed out are passed as stream 15 to the separation stage16 in which low boilers and incondensable secondary constituents 17 (oneor preferably more components from the group consisting of hydrogen,carbon oxides, nitrogen, methane, ethane, ethene, propane and propene)are separated off. The separation stage 16 can be configured as arectification column or as an absorption/desorption unit. The stream 18which comprises the C₄ products of the dehydrogenation and unreactedn-butane is fed to the dimerization reactor 19 which can have one ormore stages. The product stream 20 leaving the dimerization reactor isfractionated in the rectification column 21 to give a stream 22comprising crude 4-vinylcyclohexene and a stream 23 comprising n-butane,1-butene, 2-butene and possibly unreacted butadiene. The latter isrecirculated to the dehydrogenation reactor 11. Optionally, a substream24 can be separated off and be used in butene-based processes such asmaleic anhydride production, the oxo process, butene dimerization,trimerization and metathesis.

[0105]FIG. 2 shows the process flow diagram of a further preferredembodiment of the process of the present invention. This process differsfrom the process depicted in FIG. 1 essentially in that the dimerizationof butadiene is followed by the dehydrogenation of the4-vinylcyclohexene formed. The stream 22 comprising crude4-vinylcyclohexene obtained from the rectification column 21 isoptionally subjected to further purification in the rectification column25, with a stream of high boilers 26 being separated off. The stream 27composed of purified 4-vinylcyclohexene is fed to the dehydrogenationreactor 29 where the dehydrogenation to form ethylbenzene takes place inthe presence of hydrogen 35 and steam 28. Optionally, a side stream 41comprising 4-vinylcyclohexene can be taken off and passed to thecatalytic oxydehydrogenation to form styrene or to another use. Theproduct stream from the dehydrogenation of 4-vinylcyclohexene is cooledby means of water in the cooling and condensation unit 31 to give astream 32 of aqueous phase and a stream 33 of crude ethylbenzene. Thehydrogen formed in the dehydrogenation of 4-vinylcyclohexene, which maybe contaminated with CO, CO₂, methane, ethane and nitrogen(“dehydrogenation hydrogen”), can be partly recirculated as substream 35to the dehydrogenation reactor 29 and partly recirculated as substream34 to the dehydrogenation reactor 11. Optionally, a substream 36 of theC₄ products separated off in the separation stage 16 can be fed to afurther separation stage 37 and there separated into a stream 39comprising 1-butene, 2-butene and n-butane and a stream 38 composed ofbutadiene. A substream 40 of butadiene can be taken from the stream 38and passed to another use. The stream 39 can be recirculated at leastpartly to the dehydrogenation reactor, with a substream 42 being able tobe passed to another use.

[0106]FIG. 3 shows the process flow diagram of a further embodiment ofthe process of the present invention. This process differs from theprocess depicted in FIG. 1 essentially in that the product stream 20leaving the dimerization reactor, which comprises 4-vinylcyclohexene,n-butane, 1-butene, 2-butene and possibly unreacted butadiene, is fedinto the dehydrogenation reactor 11 in which joint dehydrogenation ofn-butane to butadiene and of 4-vinylcyclohexene to styrene takes place.Accordingly, water vapor and styrene vapor are condensed out from theproduct gas mixture 12 from the dehydrogenation in the cooling andcondensation unit 13, passed as aqueous/organic mixture 14 to the phaseseparator 21 and there separated into an aqueous phase 22 and an organicphase 23 composed of crude styrene. Optionally, part of the productmixture obtained in the dimerization reactor 19 can be fed as stream 24to a rectification column 25 where a stream 26 comprisingC₄-hydrocarbons can be separated off and passed to another use. Theremaining stream 27 comprising 4-vinylcyclohexene is fed to thedehydrogenation reactor 11.

[0107]FIG. 4 shows the process flow diagram of a particularly preferredembodiment of the process of the present invention. A feed stream 1composed of liquefied petroleum gas (LPG), which consists essentially ofpropane, n-butane and isobutane and may further comprise methane, ethaneor pentanes, is fed to a rectification column 2 and separated into astream 3 consisting essentially of propane and possibly methane andethane and a stream 4 consisting essentially of n-butane and isobutaneand possibly pentanes. In the rectification column 5, any pentanes 6present are separated off. The butane mixture 7 is separated intoisobutane 9 and n-butane 12 in the rectification column 8, withisobutane being isomerized in the isomerization reactor 10 to give ann-butane/isobutane mixture 11 which is fed back into the rectificationcolumn 8. n-Butane is fed as feed gas stream 12 into the firstdehydrogenation stage 14 in which a nonoxidative catalyticdehydrogenation of butane to 1-butene, 2-butene and butadiene takesplace. This is preferably carried out under autothermal conditions withoxygen or air being fed in as cofeed 13. The first dehydrogenation stageis preferably carried out with backmixing in the fluidized bed or withpartial recirculation of the gas, for example as described in the Germanpatent application P 102 11 275.4, which is not a prior publication. Theproduct gas stream 15 leaving the first dehydrogenation stage, whichcomprises butadiene, 1-butene, 2-butene and unreacted n-butane togetherwith water vapor and secondary constituents such as hydrogen, carbonoxides, nitrogen, methane, ethane, ethene, propane and/or propene, ispassed to a second dehydrogenation stage 17 in which oxygen or air isfed in as cofeed and an oxidative dehydrogenation of 1-butane and2-butene to butadiene takes place. The second dehydrogenation stage ispreferably carried out in a shell-and-tube reactor. The seconddehydrogenation stage can also itself be carried out in a plurality ofstages, for example in two stages. When the oxidative dehydrogenation iscarried out in two stages, the second dehydrogenation stage consists ofa first oxidative dehydrogenation stage 17 and a second oxidativedehydrogenation stage 17 a, with air or oxygen being fed in as cofeed 16or 16a in each case. The product gas stream 18 a leaving the seconddehydrogenation stage (in the case of a single-stage oxidativedehydrogenation, this is the product gas stream 18) comprises butadieneand unreacted n-butane together with water vapor and secondaryconstituents such as hydrogen, carbon oxides, nitrogen, methane, ethane,ethene, propane and/or propene, possibly small residual amounts of1-butene and 2-butene and possibly oxygen and oxygen-containinghydrocarbons (organic oxygen compounds). The product gas stream 18 a is,if appropriate after precooling in heat exchangers, cooled in thecooling and condensation unit 19, which can, for example, be configuredas a bed through which water trickles or as a falling-film condenser, tosuch an extent that water and high-boiling organic by-products such ashigh-boiling hydrocarbons and organic oxygen compounds are condensed outand are discharged from the process as stream 20. The product gasconstituents which have not condensed out are passed as stream 21 to theseparation stage 22 in which low boilers and incondensable secondaryconstituents 23 (if present in the product gas stream 18: hydrogen,carbon oxides, nitrogen, methane, ethane, ethene, propane, propene andoxygen) are separated off. The separation stage 22 can be configured asa rectification column or as an absorption/desorption unit. The stream24 which comprises the C₄ products of the dehydrogenation, unreactedn-butane and possibly organic oxygen compounds such as furan and maleicanhydride is optionally fed to a further separation stage 25 which canbe configured as a rectification column or an absorption/desorptionunit. In the separation stage 25, organic oxygen compounds and anyremaining traces of water are separated off and are discharged from theprocess as stream 26. The stream 27, which comprises butadiene andn-butane and may further comprise small proportions of 1-butene and2-butene, is fed to the dimerization reactor 28 which can have one ormore stages. The product stream 29 leaving the dimerization reactor,which may comprise as yet unreacted butadiene and small proportions of1-butene and 2-butene in addition to n-butane and 4-vinylcyclohexene, isseparated in the rectification column 30 into a stream 31 comprisingcrude 4-vinylcyclohexene and a stream 32 comprising n-butane andpossibly unreacted butadiene, 1-butene and 2-butene. The stream 32 isrecirculated to the (autothermal) dehydrogenation stage 14. The stream31 comprising crude 4-vinylcyclohexene obtained in the rectificationcolumn 30 is optionally subjected to further purification in therectification column 33 in which a stream 34 comprising high boilers isseparated off. The stream 35 comprising purified 4-vinylcyclohexene isfed to the dehydrogenation reactor 37 where the dehydrogenation toethylbenzene takes place in the presence of hydrogen 35 and withintroduction of steam 36. Hydrogen 42 can be fed in if appropriate. Ifappropriate, a side stream 49 comprising 4-vinylcyclohexane can be takenoff and passed to the catalytic oxydehydrogenation to form styrene or toanother use. The product stream 38 from the 4-vinylhexenedehydrogenation is cooled by means of water in the cooling andcondensation unit 39, giving a stream 41 composed of an aqueous phaseand a stream 40 comprising crude ethylbenzene. The hydrogen formed inthe dehydrogenation of 4-vinylcyclohexene, which may be contaminated byCO, CO₂, methane, ethane and nitrogen (“dehydrogenation hydrogen”), canbe partly recirculated as substream 42 to the (autothermal)dehydrogenation stage 14 and partly recirculated as substream 43 to thedehydrogenation reactor 37. Optionally, a substream 44 of the C₄products separated off in the separation stage 25 can be passed to afurther separation stage 45, for example a butadiene scrub (as describedin Weissermehl/Arpe, Industrielle Organische Chemie, 5th edition 1998,pp. 120/121) and separated there into a stream 47 comprising n-butaneand possibly 1-butene and 2-butene and a stream 46 comprising butadiene.A substream 48 can be separated off from the butadiene stream 46 andpassed to another use, while the remainder of the stream is fed to thedimerization reactor 28.

We claim:
 1. A process for preparing 4-vinylcyclohexene, which comprisesthe steps (A) providing an n-butane-containing feed gas stream, (B)feeding the n-butane-containing feed gas stream into at least onedehydrogenation zone and dehydrogenating n-butane to butadiene to give aproduct stream comprising butadiene, n-butane, possibly 1-butene and2-butene and possibly water vapor and other secondary constituents, (C)feeding the product stream from dehydrogenation, if appropriate afterseparating off water vapor and secondary constituents, into adimerization zone and catalytically dimerizing butadiene to give aproduct stream comprising 4-vinylcyclohexene, n-butane and possibly1-butene, 2-butene and unreacted butadiene, and (D) separating off4-vinylcyclohexene from the product stream from the dimerization andrecirculating n-butane and possibly 1-butene, 2-butene and unreactedbutadiene to the dehydrogenation zone.
 2. A process as claimed in claim1, wherein the provision of the n-butane-containing dehydrogenation feedstream comprises the steps (A1) providing a liquefied petroleum gas(LPG) stream, (A2) separating off propane and, if appropriate, methane,ethane and pentanes from the LPG stream to give a stream comprisingbutanes, (A3) separating off isobutane from the stream comprisingbutanes to give the n-butane-containing feed gas stream and, if desired,isomerizing the isobutane which has been separated off to give ann-butane/isobutane mixture and recirculating the n-butane/isobutanemixture to the isobutane separation step.
 3. A process as claimed inclaim 1 or 2, wherein the dehydrogenation of n-butane to butadiene iscarried out as an autothermal catalytic dehydrogenation.
 4. A process asclaimed in claim 1 or 2, wherein the dehydrogenation of n-butane tobutadiene comprises the steps (B1) feeding the n-butane-containing feedgas stream into a first dehydrogenation zone and catalytically,nonoxidatively dehydro-genating n-butane to 1-butene, 2-butene andpossibly butadiene to give a product gas stream comprising butadiene,n-butane, 1-butene, 2-butene and possibly secondary constituents, (B2)feeding the product gas stream comprising n-butane, 1-butene, 2-butene,possibly butadiene and possibly secondary constituents into a seconddehydrogenation zone and oxidatively dehydro-genating 1-butene and2-butene to butadiene to give a product gas stream comprising butadiene,n-butane, water vapor and possibly secondary constituents.
 5. A processas claimed in claim 4, wherein the catalytic, nonoxidativedehydrogenation of n-butane to 1-butene, 2-butene and butadiene iscarried out as an autothermal dehydrogenation.
 6. A process as claimedin any of claims 1 to 4, wherein water vapor and secondary constituentsfrom the group consisting of hydrogen, carbon lo monoxide, carbondioxide, nitrogen, methane, ethane, ethene, propane and propene areseparated off from the product stream from the dehydrogenation prior tothe dimerization.
 7. A process for preparing ethylbenzene or styrenecomprising the steps (A), (B), (C) and (D) as defined in any of claims 1to 6 and the additional step (E) feeding 4-vinylcyclohexene into afurther dehydrogenation zone and catalytically dehydrogenating it toethylbenzene or oxidatively dehydrogenating it in the presence of oxygento give styrene.
 8. A process for preparing styrene comprising the steps(A) providing an n-butane-containing feed gas stream, (F′) feeding then-butane-containing feed gas stream and a 4-vinylcyclohexene-containinggas stream into a dehydrogenation zone and jointly dehydrogenatingn-butane and 4-vinylcyclohexene in the presence of oxygen to give aproduct stream comprising styrene, butadiene, n-butane, 1-butene,2-butene, possibly ethylbenzene and further secondary constituents, (G′)separating off styrene and, if applicable, ethylbenzene and furtherhigh-boiling secondary constituents from the product stream from thedehydrogenation, (H′) feeding the stream comprising butadiene, n-butane,1-butene and 2-butene into a dimerization zone and catalyticallydimerizing butadiene to give a product stream comprising4-vinylcyclohexene, n-butane, 1-butene, 2-butene and possibly unreactedbutadiene, (I′) isolating the 4-vinylcyclohexene-containing gas streamfrom the product stream from the dimerization and feeding it into thedehydrogenation zone.
 9. A process as claimed in claim 8, wherein thejoint dehydrogenation of n-butane and 4-vinylcyclohexene is carried outin the presence of a dehydrogenation catalyst comprising a noble metalof transition group VIII together with, if desired, one or more elementsof main groups I and/or II, one or more elements of main group IIIincluding the lanthanides and actinides and/or one or more elements ofmain groups III and/or IV on a support.